1. Field of the Invention
The present invention relates to an active composition with foliated structure, consisting of a compressed support in the form of sheets and of an agent which is active towards a gas. The present invention also relates to a process for making use of a gas-solid or gas-liquid physicochemical process employing such an active composite as reaction medium.
2. Description of Related Art
A mixture of a divided material such as expanded graphite and of a solid reactant, for example a salt, or an adsorbent like a zeolite is employed in some fields like, for example, that of chemical heat pumps based on the thermicity of the reaction between a solid and a gas, or the adsorption of a gas on a solid. The mixture of expanded graphite and of this solid, the seat of a chemical reaction or of a physical adsorption, exhibits numerous advantages during a chemical reaction or a physical adsorption between the solid and a gas. Expanded graphite which is in the form of flakes or of foliated particles has a very large specific surface and allows the gas to diffuse even in a confined medium.
The substantial improvement in the reversible solid-gas reaction kinetics which is observed as a result of the mixing of the active solid with thermally expanded natural graphite, in given mass proportions and compacted into a fixed volume, is the result of an excellent permeability to the reactant gas of the stationary bed thus prepared and of a thermal conductivity accompanied by a good heat exchange coefficient at the walls. Conventionally, the preparation of such a reactant produces a homogeneous isotropic bed whose isotropic conductivity has values of 0.5 to 2 Wm.sup.-1 K.sup.-1, depending on the preparation conditions (proportion and compacting), and whose exchange coefficient at the walls lies between 50 and 150 Wm.sup.-2 K.sup.-1.
Despite these advantages the use of such mixtures in the granular states presents disadvantages because of the difficulty in obtaining really homogeneous mixtures, of their being difficult to handle and of the large volume which they occupy. Furthermore, the reactant, frequently hygroscopic, tends to absorb moisture if the mixing operation is long; a subsequent dehydration is long, or even costly, and can affect the quality of the final product, even after dehydration.
Document WO91/15292 describes an active composite in the form of a homogeneous block which includes recompressed expanded graphite subsequently impregnated with an active agent, for example a salt. This type of active composite exhibits considerable advantages when compared with the pulverulent mixtures containing expanded graphite and described above, but, nevertheless, it is difficult to produce. These difficulties relate more particularly to the distribution of the salt in the block in a uniform manner. Furthermore, the recompression of the expanded graphite typically takes place in a piston-and-cylinder unit, which gives blocks of exclusively cylindrical shape. Some applications of the actual composite call preferably for shapes other than cylinders.
The processes for the manufacture of expanded graphite are well known, in particular from U.S. Pat. No. 3,404,061. These processes, known as graphite exfoliation, include a stage of rapid expansion by heating a graphitic complex which produces a graphite powder expanded in vermicular form.